In recent years, with the development of image formation technology direct plate-making techniques without using film originals wherein letter originals and image originals are directly formed on a printing plate precursor by the scanning a narrow laser beam on the surface of printing plate precursor have been drawn attention.
Image-forming materials for such techniques include so-called thermal type positive-working lithographic printing plate precursors in which an infrared absorber included in a heat-sensitive layer reveals a light-heat conversion function to generate heat upon exposure and by the heat the exposed area of heat-sensitive layer becomes alkali-soluble, whereby a positive image is formed and so-called thermal type negative-working lithographic printing plate precursors in which by the heat generated, a radical initiator or an acid generator forms a radical or an acid and a radical polymerization reaction or an acid crosslinking reaction proceeds to insolubilize the exposed area, whereby a negative image is formed. Specifically, according to the image formation of thermal type the heat is generated from a light-heat conversion substance in the heat-sensitive layer upon exposure to laser beam and cause an image-forming reaction.
However, in case of using a grained aluminum support having an anodic oxide film formed thereon, since the heat conductivity of aluminum support is extremely high in comparison with the heat-sensitive layer, heat generated in the vicinity of the interface of heat-sensitive layer and aluminum support diffuses into the support without sufficiently using for the image formation and as a result, the following phenomenon occurs at the interface of heat-sensitive layer and aluminum support.
In the positive heat-sensitive layer, the heat diffuses into the inside of support and the alkali-solubilizing reaction proceeds insufficiently, resulting in the occurrence of remaining film in the inherent non-image area to cause a problem of decrease in sensitivity. This is an essential problem in the positive heat-sensitive layer.
Further, in the thermal type positive-working lithographic printing plate precursors, infrared absorbers having the light-heat conversion function are indispensably used. However, such infrared absorbers have problems in that they have a low solubility due to their relatively large molecular weights and in that since those adsorbed to minute openings formed by the anodic oxidation are hardly removed, the remaining film is apt to occur in a development step using an alkali developer.
On the other hand, in the negative heat-sensitive layer, the heat diffuses into the inside of support and the insolubilization of heat-sensitive layer to a developer becomes insufficient in the vicinity of the interface of heat-sensitive layer and aluminum support, resulting in the occurrence of problems in that the image is not sufficiently formed in the area wherein the image should be inherently formed and dissolved out during the development and in that even if, the image is formed, it is easily peeled off during printing.
Recently, a large number of investigations and various proposals have been made with respect to lithographic printing plate precursors, which can be mounted as they are after image exposure on a printing machine to conduct printing. For example, lithographic printing plate precursors capable of forming an image by coalescence of fine particles upon heat have been proposed.
However, such lithographic printing plate precursors have problems in that the sensitivity thereof is low because of the heat conduction to an aluminum support and in that when the coalescence of fine particles is insufficient, the strength of image area in the heat-sensitive layer degrades, resulting in insufficient press life.
In order to solve these problems, an attempt to enlarge micropores present in an anodic oxide film has been made from the standpoint of preventing the diffusion of heat generated in the heat-sensitive layer into the aluminum support.
Also, from the same standpoint, an attempt has been made for sealing the micropores by immersing an aluminum support having provided anodic oxide film on the surface of an aluminum plate in hot water or a solution containing an inorganic salt or an organic salt in hot water or exposing the aluminum support to water vapor bath as described, for example, in Patent Documents 1 and 2 described below.
However, the method of enlarging micropores present in an anodic oxide film can achieve improvements in sensitivity and press life but accompanied with degradation of staining resistance. The term “staining resistance” as used herein means a property of preventing the occurrence of stain in the non-image area in the case where printing is interrupted in the course of printing and a lithographic printing plate is allowed to stand on a printing machine and then the printing is restarted. In contrast therewith, according to the method of sealing micropores the staining resistance is improved although the sensitivity and press life are degraded. Thus, sufficiently satisfactory levels of such properties cannot be attained in these methods.
Patent Document 1: JP-A-2002-116548 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), page 8.
Patent Document 2: JP-A-2002-116549, page 2.